1. Field of the Invention
The invention relates to long stroke magnetic springs, and in particular to such springs where linearity of the force-movement function is desirable. Such springs will find use in many applications where very long life is required, and where the mechanical wear and debris-generating characteristics of typical holders for helical coil springs create a problem. The invention is of particular advantage for those machines which require not only a relatively stiff spring which is linear over the range of movement of the sprung part, but in which a motor driving force is also desired to oscillate the part over the range of spring stroke.
A particular application for spring/motor combinations is the Stirling cycle refrigerating machine, which is especially useful for cooling to very low temperatures, for example in the range of 20.degree. to 70.degree. Kelvin, or even lower. Such machines, when installed in satellites to cool infrared sensors, must be capable of operating for thousands or tens of thousands of hours without maintenance or significant loss of refrigerating capacity. The zero-wear property of magnetic springs suggests their use for this application, but the non-linearity of known spring configurations has made them unsuitable.
2. Description of the Prior Art
The magnetic bearing structures described in the literature have been thought of primarily as magnetic supports or bearings, in which an article is suspended by magnetic forces, and has only very slight movement or preferably no movement against the magnetic suspending forces. The development of these techniques is described in Magnetic and Electric Suspensions by Geary, British Scientific Instrument Research Association, 1964.
In these applications, a primary concern has been the instability forces, which tend to cause the article to be displaced in a sideways or orthogonal direction to that of the suspension direction, whenever the article is the slightest bit off center in that orthogonal direction. These forces are described in the article "Permanent Magnet Bearings and Couplings" by Jean-Paul Yonnet, IEEE Transactions on Magnetics, Volume Mag-17, No. 1, Jan. 1981, pages 1169-1173. FIGS. 2-4 of this article show all of the basic configurations used for radial and axial magnetic bearings. In general, these configurations are selected to provide the maximum bearing force, and upon analysis it will be seen that all show a relatively high nonlinearity of the relationship between spring force and movement from the center or neutral suspension location.
Another analysis of magnetic springs, again being used as bearings, is given in Paper No. V-2 "Directivity and Stability of Coaxial Permanent Magnet Systems" by Gast, Mirihmadi and Wagner at the Fifth International Workshop on Rare Earth-Cobalt Permanent Magnets and Their Applications in June, 1981 (published by University of Dayton, K1-365, Dayton, Ohio 45469. As in the article described above, the concern is primarily with the relationship between the desired spring force, and the orthogonal instability forces. Thus the article focuses on the determination of the magnitude of the unstable forces and means for compensating for these. Although a number of different configurations are discussed, none provide any suggestion as to an arrangement which will provide a relatively long stroke, linear spring force.